With development of the PCB technology, a multi-layer circuit board has an increased density between layers. Since a signal is needed to be transmitted between various layers of the multi-layer circuit board, the various layers of the multi-layer circuit board have to be electrically conductive with each other. Therefore, adjacent layers of the multi-layer circuit board need to have strictly corresponding locations for electrically conducting with each other. Deviations therebetween would affect the transmission of signals between adjacent layers.
Each layer of the multi-layer circuit board has a shape formed by printing a pattern on a copper foil layer of the circuit board by way of pattern transfer. During the pattern transfer, an exposure apparatus is used for forming a pattern on a photo-resist film on a copper foil layer by exposure.
Due to alignment errors in the exposure apparatus, during the exposure, patterns on adjacent copper foil layers which should be aligned with one another may have a registration offset from each other. After an etching process, due to the registration offset between the patterns at the conductive locations of adjacent layers, the conduction between the copper coils of the adjacent layers and thus the transmission of signal may be affected.
It is noted by the inventors of the present application that registration precision of the exposure apparatus is currently mainly determined according to an offset between positioning bores on adjacent layers after exposure. By this method, however, the registration offset of the exposure apparatus cannot be detected accurately.